There are few rock exposures beyond limited outcrops in the banks of streams, the faces of working and abandoned gravel pits, and some low coastal cliffs.
A borehole at Marchwood on the edge of the Test estuary just northeast of the national park found Jurassic and Triassic strata at depth, and penetrated what are thought to be Devonian rocks beneath.
[1] The Chalk is overlain unconformably by the clays and sands of the Reading Formation, which were deposited during the Palaeocene, the earliest part of the Palaeogene period.
The uppermost and hence youngest of all the ‘solid’ rock units within the park is the Headon Formation (forming a part of the Solent Group, which comprises clays, silts, and sands laid down in freshwater conditions.
However deposition under marine conditions is indicated by the Lyndhurst Member—a separately mappable rock unit that divides the formation into upper and lower parts.
A suite of unconsolidated materials have been deposited during the Quaternary period and include alluvial clays, silts, sands and gravels on the floors of the many smaller watercourses within the area.
Tidal flats composed of clay and silt along the Solent coast are extensive, particularly around the mouths of the Beaulieu and Lymington rivers.